As known in the prior art, in multi-cylinder dryers of paper machines, twin-wire draw and/or single-wire draw is/are employed. When employing twin-wire draw, a group of drying cylinders comprises two wires which press the web one from above and the other one from below against heated cylinder faces of drying cylinders arranged in rows. Between the rows of drying cylinders, which are usually horizontal rows, the web has free and unsupported draws which are susceptible to fluttering and may cause web breaks, in particular when the web is still relatively moist and, therefore has a low strength. For this reason, in recent years, ever increasing use has been made of the single-wire draw in which each group of drying cylinders includes only a single drying wire on whose support the web runs through the entire group so that the drying wire presses the web on the drying cylinders against the heated cylinder faces thereof, whereas on the reversing cylinders or rolls between the drying cylinders the web remains at the side of the outside curve. Thus, in single-wire draw, the drying cylinders are arranged outside the wire loop, and the reversing cylinders or rolls are arranged inside the wire loop.
In so-called normal groups with single-wire draw, known in the prior art, the heated drying cylinders are placed in an upper row and the reversing cylinders or rolls are placed in a lower row below the upper row of drying cylinders, which rows are typically horizontal and parallel to one another. In the following, when the terms "normal (dryer) group" and "inverted (dryer) group" are used, what is meant is expressly groups with single-wire draw in multi-cylinder dryers, of the type mentioned above. In an inverted dryer group, the heated drying cylinders are placed in a lower row and the reversing cylinders or rolls are placed in an upper row above the lower row of drying cylinders.
When paper is dried by means of normal groups with single-wire draw from the side of its bottom face, the drying is asymmetric and if such asymmetric drying is extended over the entire length of the forward dryer section, the drying takes place so that first the bottom-face side of the paper web is dried and, when the drying makes progress, the drying effect is also extended to the side of the top face of the paper web. Under these circumstances, the dried paper is usually curled and becomes concave, when viewed from above.
As known in the prior art, the tendency of curling of paper (or the tendency to curl) is already affected in connection with the web formation, in particular at the sheet formation stage by means of the selection of the difference in speed between the slice jet and the wire, and by means of other running parameters. For example, in the case of copying paper, by means of unequalsidedness of drying in the after-dryer, a suitable initial curl form is regulated for the sheet in order that the curling of the paper after one-sided or double-sided copying could be optimized. In the case of copying paper, the reactivity of curling, i.e., the extent to which curling occurs per unit of change in moisture content, is affected to a greater extent by means of a multi-layer structure of the paper, which is produced in connection with the web formation in the wet end.
The most recent technology related to the present invention in high-speed paper machines, in particular in fine-paper machines, is based on dryer sections in which there is single-wire draw over the major part of the length of the machine and, with a view toward controlling the tendency of curling of paper, in practice, an inverted group has also almost always been used in order that the drying may be made sufficiently symmetric in the z-direction.
In the prior art, constructions are known for an after-dryer for paper to be coated, in particular for fine paper or equivalent, in which there is first an upper cylinder and a lower cylinder and after this, one group that employs normal single-wire draw and thereafter, a necessary number of dryer groups that make use of twin-wire draw. In these applications, it is a problem that, in view of the tendency of curling of paper, the ratio of the upper and lower cylinders is inappropriate if the curling is to be regulated efficiently.
In prior art after-dryers, in particular for fine paper, in which the drying has been regulated so that the emphasis is on the lower cylinders, problems are often also encountered in relation to the moisture of air. The hood of the after-dryer, and in particular the pocket spaces of the draw(s) therein, are often excessively dry in order to control the curling. The problems described above cannot be controlled by means of an increased moisture level in the hood alone, but the moisture levels in the upper and lower pocket spaces in the draw should also be separately adjustable.
Groups of the type mentioned above for finishing of paper to be coated, in particular of fine paper, have been described, among other things, in the current assignee's Finnish Patent Application No. 950434, filed Feb. 1, 1995.